L. T. Baby

'Magic' nucleus 42Si.

Nuclear shell structures—the distribution of the quantum states of individual protons and neutrons—provide one of our most important guides for understanding the stability of atomic nuclei. Nuclei with ‘magic numbers’ of protons and/or neutrons (corresponding to closed shells of strongly bound nucleons) are particularly stable. Whether the major shell closures and magic numbers change in very neutron-rich nuclei (potentially causing shape deformations) is a fundamental, and at present open, question. A unique opportunity to study these shell effects is offered by the 42Si nucleus, which has 28 neutrons—a magic number in stable nuclei—and 14 protons. This nucleus has a 12-neutron excess over the heaviest stable silicon nuclide, and has only one neutron fewer than the heaviest silicon nuclide observed so far. Here we report measurements of 42Si and two neighbouring nuclei using a technique involving one- and two-nucleon knockout from beams of exotic nuclei. We present strong evidence for a well-developed proton subshell closure at Z = 14 (14 protons), the near degeneracy of two different (s1/2 and d3/2) proton orbits in the vicinity of 42Si, and a nearly spherical shape for 42Si.

Constraining the 6.05 MeV 0 + and 6.13 MeV 3 − Cascade Transitions in the C 12 ( α , γ ) O 16 Reaction Using the Asymptotic Normalization Coefficients

The 12C(α,γ)^16O reaction plays a fundamental role in astrophysics and needs to be known with accuracy better than 10%. Cascade γ transitions through the excited states of 16 O are contributing to the uncertainty. We constrained the contribution of the 0+ (6.05 MeV) and 3- (6.13 MeV) cascade transitions by measuring the asymptotic normalization coefficients for these states using the α-transfer reaction 6 Li(12C,d)^16O at sub-Coulomb energy. The contribution of the 0+ and 3- cascade transitions at 300 keV is found to be 1.96 ± 0.3 and 0.12 ± 0.04 u2009keVu2009b for destructive interference of the direct and resonance capture and 4.36 ± 0.45 and 1.44 ± 0.12 u2009keVu2009b for constructive interference, respectively. The combined contribution of the 0+ and 3- cascade transitions to the 12C(α,γ)16O reaction cross section at 300 keV does not exceed 4%. Significant uncertainties have been dramatically reduced.

Shell structure at N=28 near the dripline: Spectroscopy of Si-42, P-43, and S-44

Measurements of the N=28 isotones 42Si, 43P and 44S using one- and two-proton knockout reactions from the radioactive beam nuclei 44S and 46Ar are reported. The knockout reaction cross sections for populating 42Si and 43P and a 184 keV gamma-ray observed in 43P establish that the d_{3/2} and s_{1/2} proton orbits are nearly degenerate in these nuclei and that there is a substantial Z=14 subshell closure separating these two orbits from the d_{5/2} orbit. The increase in the inclusive two-proton knockout cross section from 42Si to 44S demonstrates the importance of the availability of valence protons for determining the cross section. New calculations of the two-proton knockout reactions that include diffractive effects are presented. In addition, it is proposed that a search for the d_{5/2} proton strength in 43P via a higher statistics one-proton knockout experiment could help determine the size of the Z=14 closure.

New measurement of the α asymptotic normalization coefficient of the 1 / 2 + state in O 17 at 6.356 MeV that dominates the C 13 ( α , n ) O 16 reaction rate at temperatures relevant for the s process

Background: Accurate knowledge of the 13C(α,n)16O reaction cross section is important for the understanding of the s-process in AGB stars, since it is considered to be the main source of neutrons. The sub-threshold 1/2+ state at excitation energy of 6.356 MeV in 17O has a strong influence on the reaction cross section at energies relevant for astrophysics. Several experiments have been performed to determine the contribution of this state to the 13C(α, n)16O reaction rate. Nevertheless, significant discrepancies between different measurements remain. Purpose: The aim of this work is to investigate these discrepancies. Method: An 8 MeV 13C beam (below the Coulomb barrier) was used to study the α-transfer reaction 6Li(13C,d)17O. Results: The squared Coulomb modified ANC of the 1/2+ state in 17O measured in this work is (C̃ 17O(1/2+) α−13C ) 2 = 3.6 ± 0.7 fm−1. Conclusions: Discrepancy between the results of α-transfer experiments have been resolved. However, some discrepancy with the most recent measurement using the Trojan Horse method

α -cluster asymptotic normalization coefficients for nuclear astrophysics

Background: Many important α-particle induced reactions for nuclear astrophysics may only be measured using indirect techniques due to small cross sections at the energy of interest. One of such indirect technique, is to determine the Asymptotic Normalization Coefficients (ANC) for near threshold resonances extracted from sub-Coulomb α-transfer reactions. This approach provides a very valuable tool for studies of astrophysically important reaction rates since the results are practically model independent. However, the validity of the method has not been directly verified. Purpose: The aim of this letter is to verify the technique using the 16O(6Li,d)20Ne reaction as a benchmark. The 20Ne nucleus has a well known 1− state at excitation energy of 5.79 MeV with a width of 28 eV. Reproducing the known value with this technique is an ideal opportunity to verify the method. Method: The 1− state at 5.79 MeV is studied using the α-transfer reaction 16O(6Li,d)20Ne at sub-Coulomb energies. Results: The partial α width for the 1− state at excitation energy of 5.79 MeV is extracted and compared with the known value, allowing the accuracy of the method to be evaluated. Conclusions: This study demonstrates that extracting the Asymptotic Normalization Coefficients using sub-Coulomb α-transfer reactions is a powerful tool that can be used to determine the partial α width of near threshold states that may dominate astrophysically important nuclear reaction

{alpha}-decaying states in {sup 10,12}Be populated in the {sup 10}Be({sup 14}C,{sup 10,12}Be) reaction

A search has been made for the {sup 6}He+{sup 6}He and {alpha} + {sup 8}He decay of the molecular rotational band in {sup 12}Be using the {sup 10}Be({sup 14}C,{sup 12}Be*){sup 12}C reaction at 88.5 MeV. Although the {alpha} + {sup 6}He decay of {sup 10}Be was observed in the data set there is no evidence for the breakup of {sup 12}Be. The cross-section upper limits for the {sup 10}Be({sup 14}C,{sup 6}He {sup 6}He){sup 12}C and {sup 10}Be({sup 14}C,{alpha} {sup 8}He){sup 12}C reactions are 50 and 300 nb respectively.

Low-lying states in 8B

Excitation functions of elastic and inelastic 7 Be + p scattering were measured in the energy range between 1.6 and 2.8 MeV in the c.m. An R-matrix analysis of the excitation functions provides strong evidence for new positive parity states in 8 B. A new 2 + state at an excitation energy of 2.55 MeV was observed, and a new 0 + state at 1.9 MeV is tentatively suggested. The R-matrix and time-dependent continuum shell model were used in the analysis of the excitation functions. The new results are compared to the calculations of contemporary theoretical models.

Clustering in non-self-conjugate nuclei 10Be and 18O

Clustering phenomena in 10Be and 18O were studied by means of resonance elastic scattering of α-particles on 6He and 14C. Excitation functions for α+6He and α+14C were measured and detailed R-matrix analyses of the excitation functions was performed. We compare the experimental results with the predictions of modern theoretical approaches and discuss properties of cluster rotational bands.

Structure of 8 B from elastic and inelastic 7 Be+ p scattering

Motivation: Detailed experimental knowledge of the level structure of light weakly bound nuclei is necessary to guide the development of new theoretical approaches that combine nuclear structure with reaction dynamics. Purpose: The resonant structure of 8 B is studied in this work. Method: Excitation functions for elastic and inelastic 7 Be+p scattering were measured using a 7 Be rare isotope beam. Excitation energies ranging between 1.6 and 3.4 MeV were investigated. An R-matrix analysis of the excitation functions was performed. Results: New low-lying resonances at 1.9, 2.5, and 3.3 MeV in 8 B are reported with spin-parity assignment 0 + , 2 + , and 1 + , respectively. Comparison to the Time Dependent Continuum Shell (TDCSM) model and ab initio no-core shell model/resonating-group method (NCSM/RGM) calculations is performed. This work is a more detailed analysis of the data first published as a Rapid Communication. [J.P. Mitchell, et al, Phys. Rev. C 82, 011601(R) (2010)] Conclusions: Identification of the 0 + , 2 + , 1 + states that were predicted by some models at relatively low energy but never observed experimentally is an important step toward understanding the structure of 8 B. Their identification was aided by having both elastic and inelastic scattering data. Direct comparison of the cross sections and phase shifts predicted by the TDCSM and ab initio No Core Shell Model coupled with the resonating group method is of particular interest and provides a good test for these theoretical approaches.

Developing radiation tolerant polymer nanocomposites using C60 as an additive

In nuclear facilities utilizing plutonium, polymeric materials are subjected to long-term, close-contact, and continuous α radiation exposure, which can lead to compounding material degradation and eventual failure. Herein we model the attenuation of α particles by linear-low-density polyethylene (LLDPE), polyvinyl alcohol (PVA) thin films, and C60 using Monte Carlo N-Particle Extended (MCNPX) software. The degradation of these materials was investigated experimentally by irradiating them with a beam of α particles of 5.8 MeV energy at a tandem Van de Graaff accelerator delivering a dose rate of 2.95 × 106 rad s−1 over a 7.1 mm2 sample area. Our development of a method to test α particle-induced material degradation using a tandem accelerator is significant as degradation from naturally occurring α sources (i.e. Pu, Am) occurs too slowly for these sources to be used in practical experiments. Our results show that PVA nanocomposites containing 5 wt% C60 were found to withstand about 7 times the α dose of undoped PVA films before a puncture in the film was detected. When these films were adhered to a LLDPE sheet the dual layer polymer was capable of withstanding about 13 times the dose of LLDPE and nearly twice the dose of the doped PVA thin film alone. Doping polymers with C60 is an attractive way to generate more durable, radiation tolerant materials without increasing the thickness of the material which would lead to greater waste for disposal. Thus, the results herein help to resolve a prevalent technical challenge faced in nuclear facilities that utilize polymeric materials for nuclear processing and disposal.